High performance transition metal carbide and nitride and boride based asymmetric supercapacitors

ABSTRACT

In accordance with an embodiment of the disclosure, an asymmetric supercapacitor includes a first active material with a high hydrogen over-potential and a second active material with a high oxygen over-potential. The first active material is based on a nitride, an oxynitride, a carbide, an oxycarbide, a boride, or an oxyboride of a metal selected from Groups III, IV, V, VI, and VII of the Periodic Table.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/272,185, filed Oct. 12, 2011, which claims the benefit under 35U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No.61/392,311, filed Oct. 12, 2010, the disclosures of which areincorporated herein by reference in their entireties.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grant numberF025347 awarded by the Department of Defense (DOD-TACOM). The governmenthas certain rights in this invention.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclose relates generally to asymmetric supercapacitors, and moreparticularly, to transition metal carbide and nitride based asymmetricsupercapacitors.

2. Brief Description of Related Technology

Batteries are important energy storage devices used for military andcommercial applications. While these devices can have energy densitiesexceeding 100 Wh/kg, this energy is difficult to fully access in pulsedand high power applications due to the relatively slow kineticsassociated with the redox processes of batteries.

Supercapacitors are a class of electrochemical energy-storage devicesthat could complement batteries for load-leveling or uninterruptiblepower supply applications. Referring to FIG. 1, in terms of specificenergy and specific power, supercapacitors fill the gap betweenconventional capacitors and batteries. The times shown in FIG. 1 are thetime constants of the device, obtained by dividing the energy density bythe power. Currently available supercapacitors are well suited to handlepulses of up to a few seconds. To achieve broader application, however,capacitors will have to efficiently manage longer pulses, whichtranslates to higher energy densities.

Supercapacitors have unusually high capacitances compared to traditionalcapacitors, due to their charge storage mechanisms. In addition tocharge storage during formation of an electrical double layer, a portionof the capacitance may be from fast, reversible redox reactions takingplace near the electrode surface. Supercapacitors provide higher powerthan batteries, while storing less energy. Most commercialsupercapacitors use very high-surface-area carbon-based activematerials. These materials typically yield specific capacitances of upto 200 F/g.

Some materials exploit, fast, reversible faradaic redox reactions thatoccur with the first few nanometers of the surface of the activematerial. This pseudocapacitive mechanism has been demonstrated formaterials including metal oxides and hydroxides, such as RuO₂ and MnO₂,and conducting polymers such as polyaniline and polypyrrole. HydrousRuO₂.xH₂O is a benchmark pseudocapacitive material and has been shown toyield specific capacitances ranging from 720-1300 F/g, depending on thepreparation and heat treatment conditions. Despite the high specificcapacitance of the Ruthenia-based materials, their high cost makes themunattractive for large-scale use, and therefore the commercialapplication of Ruthenia-based supercapacitors has been limited.

Despite their proven performance benefits, supercapacitors have notfound widespread commercial use, largely due to the need for higherenergy densities and lower cost. For example, the United StatesDepartment of Energy has targeted energy and power densities of 15 Wh/kgand 700 W/kg, respectively, for supercapacitors to be used forload-leveling and regenerative braking in hybrid and electric vehicles.State-of-the-art symmetric supercapacitors employing high area carbonelectrodes and non-aqueous electrolytes can reach energy densities of3-5 Wh/kg with power densities of 700 W/kg. These devices have beenhighly optimized, and only incremental gains in energy density areexpected in the future.

A type of asymmetric device architecture has been demonstrated where twodifferent types of electrode materials, for example, asupercapacitor-type electrode and a battery-type electrode are combined.The voltage windows in these devices are wider than those forconventional symmetric supercapacitors. Potential windows as wide as 2 Vhave been reported for a carbon-MnO₂ aqueous system, however, the energydensities were limited due to the moderate capacitances of carbons inaqueous electrolytes.

SUMMARY OF THE DISCLOSURE

In accordance with an embodiment of the disclosure, an asymmetricsupercapacitor includes a first electrode comprising a first activematerial and having a first operating window, wherein the first activematerial comprises a metal nitride, a metal oxynitride, a metal carbide,a metal oxycarbide, a metal boride, a metal oxyboride, and combinationsthereof, and the metal is selected from the group consisting of GroupsIII, IV, V, VI, or VII of the Periodic Table. The supercapacitor furtherincludes a second electrode comprising a second active material andhaving a second operating window, wherein first and second operatingwindows overlap by less than 60%, based on the voltage. Thesupercapacitor also includes a separator disposed between the first andsecond electrodes and an aqueous electrolyte.

In accordance with another embodiment, a method of selecting anelectrode pairing for use in an asymmetric supercapacitor includesselecting a first active material for a first electrode, the firstelectrode having a first operating window, wherein the first activematerial comprises a metal nitride, a metal oxynitride, a metal carbide,a metal oxycarbide, a metal boride, a metal oxyboride, and combinationsthereof, and the metal is selected from the group consisting of GroupsIII, IV, V, VI, or VII of the Periodic Table. The method furtherincludes determining an electrolyte pH in which the first activematerial is stable. The method also includes selecting a second activematerial for a second electrode, the second active material beingselected such that the second active material is stable at the sameelectrolyte pH in which the first active material is stable, and thesecond electrode has a second operating window, which overlaps with thefirst operating window by less than 60% based on voltage.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1a is a cyclic voltammogram of a vanadium nitride macroelectrode inan aqueous electrolyte;

FIG. 1b is a graph illustrating the specific capacity of themacroelectrode of FIG. 1 a;

FIG. 2 is a cyclic voltammogram of (a) vanadium nitride, and (b)manganese oxide in 1M KOH electrolyte using a scan rate of 50 mV/s atroom temperature. Each voltammogram was superimposed at 25 and 100cycles, and overlaid to demonstrate the stability and widened potentialwindow of about 1.8V;

FIG. 3 is a graph illustrating the constant current (0.2 mA) charge anddischarge curves for a vanadium nitride-manganese oxide button cell in1M KOH electrolyte;

FIG. 4 is a cyclic voltammogram of VN in 0.1M KOH at a scan rate of 50mVs⁻¹;

FIG. 5 is a cyclic voltammogram of W₂C in 0.5M sulfuric acid at a scanrate of 50 mVs⁻¹;

FIG. 6 is a cyclic voltammogram of Mo₂N in 0.5M sulfuric acid at a scanrate of 50 mVs⁻¹;

FIG. 7a is a cyclic voltammogram of VN in 1M KOH at a scan rate of 50mVs⁻¹;

FIG. 7b is a cyclic voltammogram of VN in 1M KOH at a scan rate of 50mVs⁻¹;

FIG. 8 is a voltage curve of a VN—MnO₂ asymmetric button cell inaccordance with an embodiment of the disclosure;

FIG. 9a is a cycling data graph for a VN—MnO₂ asymmetric button cell inaccordance with an embodiment of the disclosure;

FIG. 9b is a cycling data graph for a conventional carbon basedsymmetric button cell;

FIG. 10 is a cycling data graph for a VN—MnO₂ asymmetric button cell inaccordance with an embodiment of the disclosure;

FIG. 11 is a cycling data graph for a VN—Ni(OH)₂ asymmetric button cellin accordance with an embodiment of the disclosure;

FIG. 12 is a cyclic voltammogram of cobalt oxide in 1M KOH electrolyteat a scan rate of 50 mVs⁻¹;

FIG. 13 is a cyclic voltammogram of manganese oxide in 1M KOHelectrolyte at a scan rate of 50 mVs⁻¹;

FIG. 14 is a cyclic voltammogram of nickel oxyhydroxide in 1M KOHelectrolyte at a scan rate of 50 mVs⁻¹;

While the disclosed methods and apparatus are susceptible of embodimentsin various forms, there are illustrated in the drawing (and willhereafter be described) specific embodiments of the invention, with theunderstanding that the disclosure is intended to be illustrative, and isnot intended to limit the invention to the specific embodimentsdescribed and illustrated herein.

DETAILED DESCRIPTION

The present application describes asymmetric supercapacitors having atransition metal carbide, oxycarbide, nitride, oxynitride, boride, oroxyboride as one of the electrode materials. Asymmetric supercapacitorsinclude different active materials on each electrode. These devices canproduce voltage windows that are wider than those for symmetric devices.Since the energy density varies with the square of the voltage,increasing the voltage window in turn enhances the energy performance ofthe supercapacitor. The supercapacitors of the present disclosure mayadvantageously include a first electrode active material with a highhydrogen evolution over-potential and a second electrode active materialwith a high oxygen evolution over-potential. Using a combination of suchactive materials in a supercapacitor advantageously allows one tooperate the supercapacitor using both high oxidation and high reductionpotentials, thereby increasing the potential window of thesupercapacitor.

Asymmetric supercapacitors in accordance with embodiments of thedisclosure generally include a first electrode having a first activematerial, a second electrode having a second active material, aseparator disposed between the first and second electrodes, and anelectrolyte.

Conventional supercapacitors generally store charge using only a singlecharge storage mechanism. Most typically the charge storage mechanism isbased on electrical double layer capacitance. In contrast, theasymmetric supercapacitors in accordance with embodiments of thedisclosure can advantageously utilize two different storagemechanisms—electrical double layer capacitance and pseudocapacitance.For example, the first active material can be a pseudocapacitivematerial, while the second electrode can be an electrical double layercapacitive material. The pseudocapacitance mechanism exploits fast,reversible, faradaic redox reactions within the first few nanometers ofthe surface of the active material.

The first and second electrodes each have an operating window. As usedherein the “operating window of an electrode” refers to the maximumvoltage range in which the electrode can be stably cycled. A cyclicvoltammogram provides the operating window by plotting current asfunction of voltage. As used herein the “operating window of asupercapacitor,” also referred to herein as the “total operatingwindows” refers to the combined operating window of the electrodes,which spans from the lowest operating voltage of one of the electrodesto the highest operating voltage of one of the electrodes. As usedherein an “overlap of operating windows” refers to the voltage range inwhich both electrodes can be cycled. The percent of overlap isdetermined by dividing the voltage range of overlap by the total voltagerange of the supercapacitor (i.e., the total operating window).

The first and second electrodes can be selected to maximize the totaloperating window of the supercapacitor, that is to maximum the operatingvoltage range from negative voltages to positive voltages over which theasymmetric electrode can be cycled. By selection of an appropriate firstand second electrode active material pairing, the operating window ofthe supercapacitor may be extended beyond the performance breakdownregions that would result when using the same active material on eachelectrode. In one embodiment, the first and second electrodes areselected such that the operating windows of the electrodes do notoverlap more than 60% based on the voltage overlap. Other suitableoverlaps include less than 58%, 56%, 54%, 52%, 50%, 48%, 46%, 44%, 42%,40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5%. In one embodiment, theoperating windows can have no overlapping voltage operating portions(termed 0% overlap). For example, the electrodes can have operatingwindows that abut, but not overlap. In another embodiment, theelectrodes can have operating windows that are separated by a voltagegap.

The first electrode can have an active material with a high hydrogenevolution over-potential. For example, the active material can have ahydrogen evolution over-potential of 0.6 V or greater. The activematerial is a nitride, an oxynitride, a carbide, an oxycarbide, aboride, or an oxyboride of a metal of Groups III, IV, V, VI, and VII ofthe Periodic Table. Suitable metals include, for example, chromium,molybdenum, tungsten, vanadium, niobium, tantalum, titanium, andzirconium. For example, the active material can be vanadium nitride,vanadium carbide, molybdenum carbide, molybdenum nitride, tungstencarbide, and tungsten nitride. The active material preferably has a highspecific surface area of about 40 m²/g or greater. For example, VNhaving a specific surface area of about 40 m²/g has been measured tohave a capacity of about 140 F/g.

These materials have electrical conductivities that can be higher thanthose for transition-metal oxides, and can be produced with specificsurface areas exceeding 100 m²/g. Table 1 provides conductivities ofsome active materials, showing that the transition metal nitrides,oxynitrides, carbides, oxycarbides, borides, and oxyborides used as thefirst active material have significantly higher conductivities ascompared to carbons and transition metal oxides.

TABLE 1 Conductivities Material Conductivity (μΩ · cm)⁻¹ RuO₂ 0.0002MnO₂ 0.0005 VN 0.012 VC 0.017 Carbon 0.001

For example, VN having a specific surface area of about 100 m²/g canhave a capacity of about 300 F/g. Without intending to be bound bytheory, it is anticipated that the capacity increases linearly with theavailable surface area that is accessible by the key charge storagespecies. High specific surface area active materials can be formed, forexample, as described in U.S. Pat. No. 5,680,292, the disclosure ofwhich is incorporated herein in its entirety. These materials generallypossess good chemical stabilities and are typically inexpensive toproduce. Any other preparation methods can also be used to form the highsurface area active materials.

High surface area transition metal nitrides, oxynitrides, carbides,oxycarbides, borides, and oxyborides can be formed, for example, using aprecursor that is an oxide or chloride of the transition metal. Theprecursor can be converted to a nitride, carbide, or boride by reactingthe precursor with a reductant as the temperature is increased. Suitablereductants include, for example, ammonia, hydrazine, nitrogen, methylamine, methane, ethane, and diborane. Hydrogen and/or an inert gas canbe added to the reaction to assist the conversion of the precursormaterial to a nitride or carbide or boride. For example, the precursorcan be placed in an anaerobic chamber and subjected to a programmedincrease in temperature while ammonia gas, hydrogen gas, or nitrogengas, for example, as passed over the precursor material. The rate ofincrease in temperature may be linear (0° K/hr to 500° K/hr), forexample, or nonlinear, but should be uniform without sudden changes inrate. The reaction can be rapidly quenched after completion or held atthe final reaction temperature (500° K to 1300° K) for a period of timeand then quenched to room temperature. Optionally, the resulting nitrideor carbide can be passivated by a stream of 1% oxygen in helium, duringcooling.

In one embodiment, a high surface area nitride can be used as aprecursor material to form a high surface area carbide active material.The nitride can be chemically converted to a carbide by reacting it witha suitable reactant. Suitable reactants include, for example, methylamine, methane, and ethane. The reaction may be carried out isothermallyor in a temperature programmed manner. After completion, the reactionshould be rapidly quenched or held at the final reaction temperature fora period of time then quenched to room temperature. Hydrogen and/orinert gas may be added to assist the conversion.

In one embodiment, the precursor is coated onto an electrode substrateprior to converting the precursor to the nitride, carbide, or boride.The precursor can be dissolved in a solvent, for example, for depositiononto the electrode substrate. In another embodiment, the formed nitrideor carbide powder is dispersed in a slurry and then applied to thesubstrate and then dried. The nitride or carbide powder can be dispersedinto the slurry, for example, by stirring the nitride or carbide powderinto a solvent such as, for example, N-methylpyrrolidinone (NMP) alongwith a polymeric binder, such as for example, polyvinylidenefluoride(PVDF) and graphite. The binder and graphite are typically minorcomponents and can be included, for example, as about 10% of the totalweight of the slurry. In either embodiment, the precursor or nitride orcarbide active material can be deposited onto the electrodes substrate,for example, by dip coating, spray deposition, physical vapordeposition, or plasma arc spraying.

The second electrode can have an active material (also referred toherein as the “second active materials”) with a high over-potential foroxygen evolution. For example, the over-potential for oxygen evolutioncan be 0.6 V or greater. For a number of oxides the over-potential foroxygen evolution is a function of the enthalpy of oxygen absorption. Forexample, the second active material can be a metal oxide, such as,manganese oxide, ruthenium oxide, iridium oxide, cobalt oxide, andnickel oxyhydroxide. Cyclic voltammograms illustrating the operatingwindows of cobalt oxide, manganese oxide, and nickel oxyhydroxide areshown in FIGS. 12-14, respectively. As shown in FIG. 12, cobalt oxidehas an operating window of about −0.8 V to about 0.5 V. As shown in FIG.13, managese oxide has an operating window of about −0.05 V to about0.65 V. As shown in FIG. 14, NiOOH has an operating window of about −0.8V to about 0.3 V. Bimetallic oxides containing Pb and Fe may also beused as the second active material. The second electrode active materialshould be chosen so as to complement the redox behavior of the firstelectrode active material. For example, a Pourbaix diagram can be usedto determine suitable second electrode active materials.

As described above the second active material should be selected suchthat the overlap in operating windows of the first and second electrodesis not greater than 60% based on voltage. Suitable combinations of firstand second electrodes include for example, a vanadium nitride firstelectrode and a manganese oxide second electrode; a vanadium nitridefirst electrode and a cobalt oxide second electrode, a vanadium nitridefirst electrode and a nickel oxyhydroxide second electrode, and amolybdenum nitride first electrode and a ruthenium oxide secondelectrode.

The operating window of a supercapacitor in accordance with embodimentsof the disclosure can be greater than 1.4 V. For example, the operatingwindow can be up to about 2.2 V. Supercapacitors in accordance withdisclosure can have operating windows, for example, of about 1.5, 1.6,1.7, 1.8, 1.9, 2, 2.1, and 2.2 V.

Additionally, such increased potential windows can be achieved in anaqueous electrolyte. The use of aqueous electrolytes beneficially haslower cost, lower toxicity, and lower flammability as compared tonon-aqueous electrolytes. For example, the aqueous electrolyte can bepotassium hydroxide, sodium hydroxide, ammonium hydroxide, lithiumhydroxide, aqueous sulfuric acid, aqueous nitric acid, and aqueousphosphoric acid. The aqueous electrolyte can have a molarity of about 1Mup to about 10M. Other suitable molarities include about 1, 2, 3, 4, 5,6, 7, 8, 9, and 10M. Increased molarity advantageously has a lowerfreezing point, for example, below about −40° C., and can result inhigher charge transfer rates as a result of the high anionconcentration. Use of NaOH as the aqueous electrolyte can advantageouslydecrease the overall cost of the asymmetric supercapacitor.

The electrolyte of supercapacitors in accordance with the disclosure caneither be acidic or basic. For example, a basic electrolyte can have apH of 10 or greater. For example, an acidic electrolyte can have a pH of4 or less. Other pHs may be suitable depending on the active materialsused in the supercapacitor. The electrolyte can be selected such thatthe first and second active materials are stable in the environment(acidic or basic). For example, active materials such as vanadiumnitride, vanadium carbide, and tungsten nitride are generally stable inacidic electrolytes. Active materials such as molybdenum nitride andtungsten carbide are generally stable in basic electrolytes.

Suitable electrode substrates upon which the first active materialand/or second active materials can be coated to form the first andsecond electrodes include conductive materials such as glassy carbon,titanium, zirconium, tantalum, molybdenum, tungsten, and rutheniumoxide. Other suitable substrate materials include material selected fromGroups IV, V, VI, VII, and VIII of the Periodic Table.

In accordance with an embodiment of the disclosure, a method ofselecting a first and second electrode pairing for a supercapacitor caninclude selecting a first active material for the first electrode. Asdescribed above, the first electrode has a first operating window andthe first active material comprises a metal nitride, a metal oxynitride,a metal carbide, a metal oxycarbide, a metal boride, a metal oxyboride,and combinations thereof, and the metal is selected from the groupconsisting of Groups III, IV, V, VI, or VII of the Periodic Table. Themethod further includes determining an electrolyte pH in which the firstactive material is stable. A second active material for a secondelectrode is then selected. The second active material is selected suchthat the second active material is stable at the same electrolyte pH inwhich the first active material is stable. Selection of the secondactive material further requires selecting an electrode having a secondoperating window, which overlaps with the first operating window by lessthan 60% based on voltage. Other overlaps as described above can also beused as the selection criteria. Additionally, any of the above-describedactive materials and electrolytes can be considered for use in themethod.

EXAMPLES Example 1

An asymmetric supercapacitor having vanadium nitride as the reductionresistant electrode active material and manganese oxide as the oxidationresistant electrode active material was formed. The manganese oxideelectrode was formed as is known in the art. The vanadium nitride activematerial was formed by placing an oxide or chloride of vanadium in ananaerobic chamber and subjecting it to a programmed increase intemperature while ammonia gas or hydrogen and nitrogen gases were passedover it. The material was then passivated by a stream of 1% oxygen inhelium, during cooling. The resulting vanadium nitride had a surfacearea of about 10 m²/g. High surface area vanadium nitride powders can beproduced. The vanadium nitride was dispersed into a slurry by stirringit into a liquid such as N-methylpyrrolidinone (NMP) along with apolymeric binder such as polyvinylidenefluoride (PVDF) and graphite. Thebinder and graphite were minor components, comprising, for example,about 10% by weight of the total weight of the slurry. The slurry wascoated onto a conducting electrode substrate such as glassy carbon andoven dried.

As shown in FIGS. 1a and 1b , the vanadium nitride electrode hadelectrochemical properties suitable for use in supercapacitors,including, for example, high capacitance as well as high stability. FIG.1a shows the high stability of a vanadium nitride electrode,illustrating multiple overlapping scans without significant variation,which corresponds to stable charging and discharging of the capacitor.

By combining the vanadium nitride electrode with a manganese oxideelectrode, the voltage window, and correspondingly the capacitance, ofthe supercapacitor can be expanded. As illustrated in FIG. 2, thecombined use of the two electrodes allows for a widened potential windowof about 1.8V when used in an aqueous electrolyte. Specifically, a 1MKOH electrolyte was used to test the potential window. Advantageously,the potential window was expanded beyond the typical maximum voltage of1V for aqueous electrolytes.

The vanadium nitride and manganese oxide electrodes were incorporatedinto a CR2032 button cell. Referring to FIG. 3, the button cell yieldedan energy density of 4.4 Wh/kg at 1.6 V. The potential window was widerthan the thermodynamic limit for an aqueous system. Without intending tobe bound by theory, it is believed that the increased potential windowis the result of the high hydrogen over-potential exhibited by thevanadium nitride under negative polarization and the high oxygenevolution over-potential exhibited by the manganese oxide. The cellcould be cycled for more than 1000 times.

The extent of widening of the potential window for an asymmetriccapacitor may be determined by selection of the active materialpairings. In some examples, the pairing may be constrained by having adesired overlap in potential windows of each individual active material.The overlap provides for efficient transfer between reduction andoxidation states without entering a region of performance (or activematerial) breakdown. The extent of such overlap may be adjusted based onactive material type and, where desired, to maximize the width of theresulting asymmetrical pairing potential window.

Example 2

The electrode materials tested were VN, VC, Mo₂C, Mo₂N, W₂C and W₂N andthe electrolytes were aqueous solutions of KOH and H₂SO₄. Cyclicvoltammetry was used to identify stable electrode and electrolytesystems and also to determine the stable operating voltage window. Thecyclic voltammetry experiments were performed using a microelectrode asthe working electrode. The transition-metal carbides and nitrides wereloaded onto the tip of a gold microelectrode by abrasive adhesion.

A three-electrode system was used to perform cyclic voltammetry with aPt flag as the counter electrode (CE). The reference electrode (RE) wasHg/HgO in KOH and Hg/HgSO₄ in H₂SO₄. FIGS. 4-6 show cyclic voltammograms(overlay of the 25^(th) and 100^(th) scan within a potential range) forVN, W₂C and Mo₂N at a scan rate of 50 mV/s. One hundred scans werecarried out within a potential range and then the range was expanded inorder to determine the widest stable potential range. Table 2 summariesthe results of the screening experiments and their measured capacitance.The capacitance in Table 1 was measured at a scan rate of 2 mVs⁻¹.

TABLE 2 Results for Electrochemical Characterization by CyclicVoltammetry Stability Window Capacitance (V) Material (F/g) 1.1 (KOH) VN210 0.8 (KOH) VC 2.6 0.8 (H₂SO₄) Mo₂N 346 0.7 (H₂SO₄) W₂C 79 0.8 (KOH)W₂N 25

Table 3 summarizes the stability results of the screening experiments.

TABLE 3 Stability Results Stability Window (V) Material KOH ElectrolyteH₂SO₄ Electrolyte VN 1.2 Unstable VC 0.7 Unstable Mo₂N Unstable 0.8 Mo₂CUnstable Unstable W₂N 0.7 Unstable W₂C Unstable 0.8

In order to determine the electrolyte species contributing to thepseudocapacitive behavior in VN, preliminary experiments were carriedout on the VN—KOH system. The electrolyte ions K⁺ and OH⁻ were isolatedand paired with redox inactive counter ions as shown in Table 4.

TABLE 4 Isolation of KOH Ions by Forming Pairs with Inactive CounterIons Anion OH⁻ (CF₃SO₃)⁻ Cation (Hydroxyl) (Triflate) K⁺ K⁺OH⁻ K⁺(CF₃SO₃)⁻ (Potassium) (Potassium Hydroxide) (Potassium Triflate)(C₂H₅)₄N⁺ (C₂H₅)₄N⁺ OH⁻ (C₂H₅)₄N⁺(CF₃SO₃)⁻ (TEA) (Tetraethylammonium(Tetraethylammonium Hydroxide, TEA-OH) Triflate, TEA-Triflate)

The counter ions, tetraethylammonium (TEA)⁻ and triflate⁺, were chosenas they are known to be redox inactive. Cyclic voltammetry for VN ineach of these four electrolytes in aqueous 0.1M solutions, wereconducted. VN was scanned at a scan rate of 50 mVs⁻¹. These resultsshowed that the redox behavior of VN in TEA-OH and K—OH were similar.The peaks seen in the case of K-triflate do not coincide with thoseobserved for VN in K—OH. The hydroxyl appears to be the active species,not the solvent or other ions in the solvent.

Given these findings and with a view towards reducing the supercapacitorcost, NaOH may be used as an alternative to KOH. The use of NaOH couldreduce costs associated with the electrolyte by about 50%. FIGS. 7a and7b illustrate voltammograms for VN in 1M KOH and 1M NaOH, respectively.The results were nearly identical, suggesting that the electrochemistryand amount of energy stored for these systems is comparable.

Example 3

A VN—MnO₂ supercapacitor was tested to determine the operating window.Referring to FIG. 8, the supercapacitor was demonstrated to have anoperating window in excess of 2V (about 2.2V). The supercapacitorincluded VN and MnO₂ electrodes in 1 M KOH electrolyte. The VN had amodest surface area of about 40 m²/g and a capacity of 140 F/g. Thesupercapacitor was assembled using a CR2032 button cell. The cellyielded energy densities up to about 8.6 Wh/kg (based on the activematerial) at 2V. The potential window was wider than the thermodynamiclimit for an aqueous system because of the high hydrogen evolutionover-potential exhibit by VN under negative polarization and the highoxygen evolution potential for MnO₂ as the positive electrode.

Referring to FIG. 9a , cells were cycled from full charge to fulldischarge with a current of 1 A/g active material for more than 1000cycles and demonstrated high energy densities which were comparable tohighly optimized carbon systems (FIG. 9b ). Cycling in FIG. 9a wasperformed at a constant current of 10 mA (0.01-2V). The carbon cell ofFIG. 9b was a carbon based symmetric button cell in 1.5 Mtetraethylammonium tetrafloroborate (TEA-BF₄) in acetonitrile as theelectrolyte. Cycling in FIG. 9b was performed at a constant current of 5mA (0.01-2.7V) at room temperature. This data suggests that furtheroptimization of VN—MnO₂ systems, such as by improving the specificsurface area of the active material, will results in supercapacitorswhich exceed the performance (energy density) of current, highlyoptimized carbon systems. In contrast to the carbon system, the VN—MnO₂can achieve these high energy densities while advantageously using anaqueous electrolyte.

FIG. 10 similarly illustrates the cycling of a VN—MnO₂ cell. The cellincluded a first electrode having 2.9 mg of VN (having a specificsurface area of 1.5 mg/cm²) as the first active material and a secondelectrode having 6.5 mg of MnO₂ (having a specific surface area of 3.3mg/cm²) as the second active material. The mass ratio MnO₂/VN was 2.24.The cell was cycled from 0.1 V to 1.7 V at a constant current of 5 mA.

FIG. 11 illustrates the cycling of a VN—Ni(OH)₂ cell. The cell wascycled form 0.1 V to 1.7 V at a constant current of 5 mA. Both FIGS. 10and 11 further demonstrate that the supercapacitors in accordance withembodiments of the disclosure are capable of exceeding the energydensities achieved by conventional highly optimized carbon systems (FIG.9b ).

Asymmetric supercapacitors in accordance with embodiments of thedisclosure can be used in a variety of applications, including, forexample, military applications. For example, the supercapacitors can beused for powering devices for the modern soldier and exoskeleton systemfor Human Universal Load Carrier (HULC), powering electromagnetic armor,and extended range vehicles. The supercapacitors of the disclosure canalso be used in the automotive industry, for example, in accelerationboost and regenerative braking for cars and trucks. The supercapacitorsof the disclosure can also be used in advanced auxiliary power units andbuffering for peak power associated with renewable energy sources, suchas wind turbines.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

1. An asymmetric supercapacitor comprising: a first electrode comprisinga first active material and having a first operating window between aminimum voltage and a maximum voltage, wherein the first active materialcomprises a metal nitride, a metal oxynitride, a metal carbide, a metaloxycarbide, a metal boride, a metal oxyboride, or a combination thereof,and wherein the metal is selected from the group consisting of GroupIII, IV, V, VI, or VII of the Periodic Table; a second electrodecomprising a second active material and having a second operating windowbetween a minimum voltage and a maximum voltage, wherein the first andsecond operating windows overlap by less than 15% between the minimumvoltage of the first electrode and the maximum voltage of the secondelectrode; and an aqueous electrolyte.
 2. The asymmetric supercapacitorof claim 1, wherein the first and second operating windows overlap byless than 10%.
 3. The asymmetric supercapacitor of claim 1, wherein thefirst and second operating windows overlap by less than 5%.
 4. Theasymmetric supercapacitor of claim 1, wherein the first active materialis selected from the group consisting of vanadium nitride, vanadiumcarbide, molybdenum nitride, molybdenum carbide, tungsten nitride,tungsten carbide, and combinations thereof.
 5. The asymmetricsupercapacitor of claim 4, wherein the second active material isselected from the group consisting of manganese oxide, ruthenium oxide,iridium oxide, cobalt oxide, nickel oxyhydroxide, and combinationsthereof.
 6. The asymmetric supercapacitor of claim 5, wherein theaqueous electrolyte is selected from the group consisting of potassiumhydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide,sulfuric acid, nitric acid, phosphoric acid, and combinations thereof.7. The asymmetric supercapacitor of claim 1, wherein the first activematerial has a specific surface area of 40 m²/g or greater.
 8. Theasymmetric supercapacitor of claim 1, wherein the asymmetricsupercapacitor has a total operating window between the minimum voltageof the first electrode and the maximum voltage of the second electrode,and wherein the total operating window is greater than 2 Volts.
 9. Anasymmetric supercapacitor comprising: a first electrode comprising asubstrate and a coating layer, the coating layer comprising a metalnitride, a metal oxynitride, a metal carbide, a metal oxycarbide, ametal boride, a metal oxyboride, or a combination thereof, wherein themetal is selected from the group consisting of Group III, IV, V, VI, orVII of the Periodic Table; a second electrode comprising a metal oxide;and an aqueous electrolyte.
 10. The asymmetric supercapacitor of claim9, wherein the metal oxide has a high over-potential for oxygenevolution.
 11. The asymmetric supercapacitor of claim 9, wherein themetal of the metal oxide in the second electrode is different than themetal in the coating layer of the first electrode.
 12. The asymmetricsupercapacitor of claim 9, wherein the first active material is selectedfrom the group consisting of vanadium nitride, vanadium carbide,molybdenum nitride, molybdenum carbide, tungsten nitride, tungstencarbide, and combinations thereof.
 13. The asymmetric supercapacitor ofclaim 12, wherein the metal oxide of the second electrode is selectedfrom the group consisting of manganese oxide, ruthenium oxide, iridiumoxide, cobalt oxide, nickel oxyhydroxide, and combinations thereof. 14.The asymmetric supercapacitor of claim 9, wherein the coating layerfurther comprises a solvent and a polymeric binder.
 15. The asymmetricsupercapacitor of claim 14, wherein the solvent isN-methylpyrrolidinone, and the polymeric binder is polyvinylidenefluoride.
 16. The asymmetric supercapacitor of claim 9, wherein thefirst electrode has a first operating window between a minimum voltageand a maximum voltage, wherein the second electrode has a secondoperating window between a minimum voltage and a maximum voltage, andwherein the first and second operating windows overlap by less than 15%between the minimum voltage of the first electrode and the maximumvoltage of the second electrode.
 17. The asymmetric supercapacitor ofclaim 9, wherein the aqueous electrolyte is selected from the groupconsisting of potassium hydroxide, sodium hydroxide, ammonium hydroxide,lithium hydroxide, sulfuric acid, nitric acid, phosphoric acid, andcombinations thereof.
 18. An asymmetric supercapacitor comprising: afirst electrode comprising vanadium nitride and having a first operatingwindow between a minimum voltage and a maximum voltage, wherein thefirst active material comprises a metal nitride, a metal oxynitride, ametal carbide, a metal oxycarbide, a metal boride, a metal oxyboride, ora combination thereof, and wherein the metal is selected from the groupconsisting of Group III, IV, V, VI, or VII of the Periodic Table; asecond electrode comprising nickel oxyhydroxide and having a secondoperating window between a minimum voltage and a maximum voltage,wherein the first and second operating windows overlap by less than 15%between the minimum voltage of the first electrode and the maximumvoltage of the second electrode; and an aqueous electrolyte.
 19. Theasymmetric supercapacitor of claim 18, wherein the asymmetricsupercapacitor has a total operating window between the minimum voltageof the first electrode and the maximum voltage of the second electrode,and wherein the total operating window is greater than 2 Volts.
 20. Theasymmetric supercapacitor of claim 18, wherein the first electrodecomprises a substrate and a coating layer, and the coating layercomprises the vanadium nitride.